The invention relates to an optical communications device, and more particularly, to a variable optical fiber attenuator adapted for use in a compression connection device of an optical communications system.
Optical communications systems include a variety of devices interconnected with optical fibers (e.g., light sources, photodetectors, switches, amplifiers, filters, and so forth). Signals are transmitted along an optical fiber by internal reflection, e.g., the core of the fiber is fabricated with a material having a higher index of refraction than the cladding surrounding the core so that light signals launched into the fiber will be guided along the core by internal reflection. For typical transmission applications, a fiber optic core is fabricated with about 3% GeO2/SiO2.
In designing optical communications systems, lengths of optical fibers are terminated and connected to other fibers, devices, or termination ports. For example, in a wavelength division multiplexed optical network system, optical fibers may be used to connect a plurality of transmitters to a multiplexer, an optical switch, a demultiplexer, and a receiver, and signals may be sent over the length of the fibers between the multiplexer and receiver, with various connections being made along the transmission path. A widely-used connector comprises the ST(copyright) connector, ST(copyright) being a registered trademark of ATandT Corp., now Lucent Technologies, Inc., the assignee of the present application. The ST(copyright) connector and various other connectors are known and used in the field.
Many challenges inhere in making these connections while maintaining efficient signal transmission, and in particular, in providing connectors that provide a desired level of attenuation in the transmitted signal. Such challenges include aligning the fiber cores, index-matching the materials to achieve low reflectance, addressing thermoplastic creep issues, and achieving and varying the levels of signal attenuation. For example, fiber cores typically have small diameters (e.g., they may be as small as 8 microns), and thus, it is difficult to precisely align the cores of two fibers.
As mentioned, index-matching is a concern in the connection devices. An index differential between interfacing components will affect the reflectance of the transmitted signal. The closer index-matching there is between interfacing materials, the less reflectance there will be. Reflectance can be calculated as a function of the refractive index differential of interfacing materials, i.e.,
Reflectance (dB)=10 log[(noxe2x88x92ni)2/(no+ni)2],
where no and ni are the refractive indices of two interfacing materials. Thus, for a glass-to-air interface (no≈1.0 for air, ni≈1.46 for glass), the reflectance is about 14.6 dB. For two materials with only slightly different indices of refraction (e.g., ni≈1.46 vs. 1.47), the reflectance is xe2x88x9249.3 dB. Optimally for high performance systems, reflectance generated by an optical connection should be less than xe2x88x9250 dB.
Another concern that affects the selection of materials used to make connection devices relates to glass transition temperatures and thermoplastic creep. In operation, an attenuator of a compression connection device will be pressed against an optical fiber for long periods of time and subjected to large compressive forces. These forces create indentations on the attenuator surface. If the attenuator is left in place, the indentation will not affect the system performance. However, the attenuators are usually repeatedly removed and re-inserted into the connector device. Where an indentation is formed on an attenuator, it is unlikely that the attenuator, when placed back into a connector, will rest against the fiber in exactly the same way as in the initial connection. For example, upon reconnection, the fiber end face may hit a lip of a depression. An air gap thus may be formed which would negatively affect the attenuation and back-reflection properties of the device, for as indicated above, an air-fiber interface produces a large refractive index differential.
The extent of creep and therefore, the extent or size of the surface deformations will depend on the applied force, the use temperature, and the glass transition or heat distortion temperatures of the materials used to fabricate the connector elements. The heat distortion temperature reflects the temperature at which significant distortions occur. In other words, the glass transition or heat distortion temperature is the temperature at which the material""s behavior changes from a high modulus, glassy response to a low modulus, rubbery response. The glass transition temperature is typically measured at 264 psi according to standards known in the field as ASTM D648. The higher the heat distortion temperature of the materials, the more resistant the material will be to deformation. Materials having glass transition temperatures below room temperature are identified herein as xe2x80x9ccompliantxe2x80x9d materials. The term xe2x80x9ccompliancexe2x80x9d is used herein to refer to a material""s susceptibility to deformation upon compression. Thus, a high compliant material is one that exhibits a rubbery, deformable response at room temperature, and a low (or non-) compliant material is one that is more resistant to deformation and exhibits significantly smaller indentations at room temperature over relatively long periods of time. The period of time during which an indentation would be resisted is temperature dependent. At ambient temperatures, a low-compliant material will resist indentations for a period of years. At elevated temperatures, this period may be reduced to months.
To reduce the likelihood of thermoplastic creep, traditionally efforts had been directed toward developing very stiff, non-compliant materials (e.g., having glass transition temperatures above room temperature), so that compression of the materials would not produce indentations. It has been taught that the heat distortion or glass transition temperature of the attenuator materials should be greater than about 80xc2x0 C. and even more preferably above 100xc2x0 C. For example, U.S. Pat. No. 5,082,345 to Cammons et al., issued Jan. 21, 1992 and assigned to the present assignee, describes a connection device that uses polymethylmethacrylate (PMMA) to fabricate an attenuator element. PMMA is a stiff (non-compliant) material resistant to indentations; it is used to make a disc-shaped attenuator element which is disposed between opposing optical fibers and held in place in a connector sleeve by spring-loaded plugs (see, e.g., Cammons FIG. 2, col. 7, 1. 48-65). However, PMMA has an index of refraction of 1.49, whereas a fiber optic core typically has a refractive index of about 1.451 at 25xc2x0 C. and 1300 nm. This refractive-index differential correlates to an attenuator portion producing xe2x88x9240 dB reflectance which is suitable for many applications, but less than optimal for high performance optical fiber systems.
Additionally, in the Cammons design, levels of attenuation are not tuned in situ. Light emerging from a fiber incident on the attenuation element is scattered in the forward direction toward an opposing fiber. The magnitude of the light accepted by the opposing fiber is determined by the acceptance angle that is established by the physical separation between the two fibers. Elements with known thicknesses and known attenuation levels are provided in pre-assembled optical fiber buildouts or couplers. Consequently, Cammons introduces a certain, predetermined gap between the connected fibers and provides for a fixed level of attenuation.
An improved optical terminator having closely-index matched materials is described in U.S. Pat. No. 5,619,910, issued Apr. 8, 1997 to King et al, titled xe2x80x9cOptical Terminatorxe2x80x9d (assigned to the present assignee and incorporated herein). The device of the King patent uses a ferrule having an index of refraction of about 1.45xc2x10.01, and advantageously it achieves an attenuation of less than 50 dB below the incident signal (e.g., at col. 8, 1. 25-33). In achieving index matching, King uses thermoplastic polymers and raises their heat distortion temperatures by blending them with other compounds such as polyimide, polyvinylidene fluoride, and polymethylpentene polymers. Thus, King follows the teaching that materials having high (above room temperature) heat distortion or glass transition temperatures are preferred (e.g., col. 6, 1. 56-57). However, blending the materials with compounds to increase their heat distortion properties may unacceptably alter the refractive indices of the materials, and vice-versa. Also, there are a limited number of thermoplastic polymers that may be used in optical connections meeting all the desired criteria. Furthermore, the King device is configured as a terminator device for which in situ tuning of the attenuation level is not pertinent.
Previous attenuators that allow for in situ tuning typically rely upon adjustable air gaps as described, for example, in U.S. Pat. No. 5,734,778, xe2x80x9cVariable Attenuator Connector,xe2x80x9d issued Mar. 31, 1998 to Loughlin et al, and U.S. Pat. No. 5,066,094, xe2x80x9cVariable Optical Fiber Light Attenuator,xe2x80x9d issued Nov. 19, 1991 to Takahashi. In both the Loughlin and Takahashi cases, mechanical assemblies (e.g., with use of screws and nested thread assemblies) are used to vary the size of an air gap formed between the end faces of two aligned fibers, and the change in the space between the fibers (the size of the air gap) produces a change in attenuation level. However, as noted above, there is a large refractive index differential at a fiber/air interface. Thus, back reflection is a concern with these devices, making them disadvantageous for many telecommunications applications and/or requiring additional features designed to address back reflection problems.
As may be appreciated, those in the field of communications systems continue to seek to develop new components to improve performance, increase efficiency, and reduce cost. In particular, there remains a need for an optical attenuator element that is index-matched to optical fibers to achieve low reflectance, and that addresses thermoplastic creep and is tunable to a wide range of attenuation levels, allowing for in situ tuning of the attenuation level.
Summarily described, the invention comprises an attenuator assembly of a connection device for use in an optical communications system that is tunable in that the level of attenuation can be varied in situ. Tunability is achieved with use of an elastomeric attenuator element disposed between the end faces of at least two optical fibers. With the invention, a mechanism is provided for changing the thickness of the attenuator element in situ in the connection device, e.g., by compressing or enabling the expansion of the attenuator element, which thereby varies the level of attenuation of the signal. Preferably, the attenuator element is fabricated with silicone and the thickness-adjusting mechanism comprises at least one spring-loaded plug adjacent at least of the optical fiber end faces.